1. Field of the Invention
The present invention relates to a high-power semiconductor laser that is suitable as the light source for a distance measurement system or device, and a light-sensing device using this semiconductor laser.
2. Description of Prior Art
There have recently been experiments into using semiconductor lasers as light sources in the field of distance measurement, in applications such as measuring the spacing between vehicles. When a semiconductor laser is used to measure a distance, high-power pulses of light are emitted toward a target, and the distance to the target is calculated from the time taken from the emission of the pulsed light until the reception of light reflected from the target.
The characteristics demanded of a semiconductor laser capable of performing in this manner are (a) an ability to lase at a high power level of, for example, a maximum output of at least 50 W, (b) a single-peaked far-field pattern, (c) no delay in the lasing, and (d) emission of light in a wavelength region that is not easily affected by sunlight. Of these characteristics, the output and lasing wavelength of a semiconductor laser greatly influence the capabilities of the distance measurement apparatus. In other words, the output of the semiconductor laser affects the distance travelled by the light, so that the distance the light travels increases as the light output increases, and thus the measurable range also increases.
Another point concerning the lasing wavelength of the laser is that it is advantageous to have a longer wavelength. If, for example a distance measurement apparatus is used as a vehicle-spacing measurement apparatus, sunlight would interfere as noise in this distance measurement apparatus. In order to avoid the problem of such noise, generally a bandpass filter that passes only light of the same wavelength of the laser light is placed in front of the light-receiving sensor, in order to cut out as much of the sunlight as possible. Nevertheless, it is impossible to prevent sunlight of the same wavelength as the laser light from being incident on the light-receiving sensor. The energy density distribution of sunlight reaches as low as the long wavelengths of the infrared region. This means that the effects of sunlight would be less likely to become a problem if the lasing wavelength of the laser could be made longer, and thus noise due to sunlight would be reduced.
However, the maximum output of a popular semiconductor laser is approximately 20 W. Achieving a greater output causes problems in that the output becomes thermally saturated and thus efficiency drops, or the output drops because of an increase in injection current density, or the edge surfaces of the laser become damaged. If a semiconductor laser is used as a light source of a distance measurement apparatus, the distance that can be measured with a laser of a maximum output of approximately 20 W is less than 100 meters, and thus highly accurate measurement is difficult when the object to be inspected is beyond that distance.
Techniques disclosed in, for example, Japanese Patent Application Laid-Open Nos. 59-198377 and 61-149876, are intended for application to distance-measuring systems that use semiconductor lasers. Each technique involves the provision of a plurality of light-transmitting devices with beams of differing angles of projection for far and near distances as the radar for measuring vehicle spacing, in order to enable measurement over a wide range from far to close distances. In the technique disclosed in the former patent application, the light from one light source is split by optical fibers to form a plurality of light sources, or a plurality of independent light sources are used, and lenses with differing angles of projection corresponding to the light sources are provided independently to implement far and near illumination. The technique disclosed in the latter patent application allocates the far and near illumination to a plurality of light sources and lenses for adjusting the angles of projection of the light sources that are arranged independently.
Each of these techniques is advantageous in that it has the function of reducing dead angles when used in a radar that can measure over a wide range from far to close distances. However, a plurality of light sources is necessary in the above described structures, and an independent optical system must be provided for each light source, which not only makes the apparatus complicated, but difficult to make it compact.